Field of the Invention
The present invention relates to a vibration-type actuator, a driving method for the vibration-type actuator, and an electronic apparatus equipped with the vibration-type actuator.
Description of the Related Art
There is a known vibration-type actuator that makes a vibration body be in pressure contact with a driven body and moves the vibration body and driven body relatively by a vibration excited in the vibration body (see Japanese Laid-Open Patent Publication (Kokai) No. 2004-320846 (JP 2004-320846A)).
FIG. 15 is a perspective view schematically showing a configuration of a well-known vibration-type actuator 100. The vibration-type actuator 100 has a vibration body 102 and a driven body 103. The vibration body 102 has an elastic body 102b, two projections 102c disposed on one side of the elastic body 102b, and a piezoelectric device 102a joined to the elastic body 102b at the side opposite to the side on which the projections 102c are disposed.
The two projections 102c are arranged at a predetermined interval in an X-direction. The front ends (upper surfaces) of the projections 10c are in pressure contact with the driven body 103. The elastic body 102b and the two projections 102c are made from metal material, such as stainless steel. The piezoelectric device 102a is adhered to the elastic body 102b with adhesive, for example. A driving circuit (not shown) applies two-phase alternating voltage to the piezoelectric device 102a, and accordingly, vibrations in two bending vibration modes are excited in the vibration body 102. Then, the elliptic motion in the Z-X plane is generated at the front ends of the projections 102c by combining the vibrations excited in the two bending vibration modes. Since the front ends of the two projections 102c are in pressure contact with the driven body 103, the driven body 103 receives friction driving force by elliptic motions of the two projections 102c. Accordingly, the vibration body 102 and the driven body 103 are moved relatively in the X-direction.
A vibration-type actuator drivingly rotates a driven body using the plurality of vibration bodies 102. For example, the three vibration bodies 102 are arranged on the same circumference at regular intervals so that a line that connects two projections 102c of each vibration body coincides with a tangent of the circumference. Accordingly, a disk-shaped or ring-shaped driven body that is in pressure contact with the two vibration bodies 102 rotates (see Japanese Laid-Open Patent Publication (Kokai) No. 2012-5309 (JP 2012-5309A)).
However, the vibration-type actuator using the vibration body 102 may cause an abnormal noise called a “squeal” in a high frequency range at a low driving speed, when the frequency of alternating voltage is swept so as to approach a resonance frequency in a vibration mode used for driving from a frequency higher than the resonance frequency. Specifically, the frictional force between the two projections 102c and driven body 103 causes an unnecessary vibration as a self-excited vibration while sweeping the high frequency side in the driving frequency range. When the unnecessary vibration occurs, the frequency difference between the frequency of the unnecessary vibration and the driving frequency becomes audible, which causes the abnormal noise. Then, when the frequency difference between the resonance frequency in the vibration mode used for driving and the driving frequency is approximately coincident with the frequency difference between the frequency of the unnecessary vibration occurred and the driving frequency, while the driving frequency is further swept toward the low frequency side, the abnormal noise becomes large and the driving also becomes unstable.